The formation of polymeric films directly on rubbing surfaces to reduce wear

Abstract

A new concept—the reduction of wear by the use of particular compounds capable of forming protective polymeric films directly on rubbing surfaces—is presented; and the idea is supported by experimental data obtained in high contact stress systems with a variety of compounds. In addition, this idea has led to the development of several new and potent additives for reducing wear and scuffing. According to this concept, the potential polymer-forming compound (or compounds) is dissolved or mixed at low concentrations in a carrier ( e.g., a liquid hydrocarbon). Due to the high surface temperatures in regions of greatest contact and possibly to the added catalytic action of certain freshly exposed surfaces, very thin protective polymeric films will form in these areas. The first step may involve concentration of the polymer-former on the solid surface by adsorption. In a dynamic system, the polymer films are continually being formed and worn away. However, the system tends to have a built-in control; the very formation of the polymer film will tend to reduce contact and friction and therefore the rate at which the film continues to form. As the polymer film is physically worn away, friction and surface temperatures will tend to increase, thereby causing the rate of film formation to increase.The interposed film is a deposited one and may be relatively thick.Its basic function is to reduce contact and adhesion between solids. Evidence in support of the in situ polymer film mechanism includes (a) the superiority of polymer-forming compounds over closely-related non polymer-formers in a variety of boundary lubrication tests, (b) indications of localized and adherent surface films, (c) the existence and solvent removal behavior of pronounced beneficial carryover effects, (d) the results of mechanism studies in a dynamic sliding system with radioactively-labeled compounds, and (e) the overall success of the approach in developing new classes of effective additives for reducing wear, scuffing, and surface damage. An outstanding example of the application of this concept to a real problem was demonstrated by the striking effectiveness of partial ( e.g., mono-) esters made by reacting long-chain C 36 dimer acids with glycols ( e.g., ethylene glycol). These compounds are believed to act by the formation of polyester films as follows: HOOC-R-COO-C 2H 4OH→ ▪HO[-OC-R-COO-C 2H 4O] H + H 2O↑ It was found that useful polymer-formers may be of the condensation type ( e.g., to form polyesters, polyamides) or of the addition type; but the condensation or step-reaction approach appears to be more effective in the systems investigated. The picture is complex, however, and not all polymer-formers function. Possible reasons for this are discussed.